The present invention relates generally to fluid processing and sensors used for measuring process variables, such as pressure, temperature, level and flow. In particular, the invention relates to thermowells used to sense temperature of flowing fluids. Although, the invention may be applied to any probe having a sensor disposed within a housing configured for insertion into a process fluid flow. Thermowells conventionally comprise a tube that extends through a fluid conduit wall, such as a pipe, so that the exterior of the tube is in thermal communication with the process fluid. A temperature sensor, such as a thermocouple or resistance temperature detector (RTD), is in thermal communication with the interior of the tube to measure the temperature of the process fluid. Wiring extending through the tube connects the temperature sensor to transmitter electronics, which are typically in electronic communication with a process control network through an appropriate wired or wireless network. As such, temperature readings from the temperature sensor can be processed and communicated to a workstation at a process control room.
Within the fluid conduit, the sensor tube is exposed to forces generated by flow of the process fluid. In particular, the sensor tube is subject to a number of stress factors including flow-induced vibrations. Flow-induced vibrations typically arise as a result of vortex shedding and other turbulent flow field effects, which generate periodically alternating forces that excite the resonance of the sensor tube. These forces cause the tube to oscillate back and forth or vibrate, increasing mechanical stress and reducing service life for both the sensor tube and its associated sensor. Flow-induced vibrations are particularly problematic when they occur near a natural resonant frequency, producing forced resonant oscillations that can potentially result in catastrophic failure, such as from repetitive fatigue stress. Even relatively small oscillations can also be an issue, particularly when combined with other stresses such as high drag forces or static pressure gradients, or with corrosion, fatigue, or erosion of the sensor tube structure.
Guidelines, such as those described in ASME PTC 19.3, are established for flow rates at which particular thermowells can be used to avoid resonance frequencies that generate large vibration loads. Problems associated with sensor tube vibrations have previously been addressed by increasing the strength of the sensor tube. This approach requires thicker tube walls or specialized construction, which increases cost, expands the size and weight envelope of the device, decreases sensitivity and increases response time. Alternatively, sensor tubes have been configured to reduce vortex shedding (which causes flow-induced vibrations) such as by including flow disrupting features that force the separation of the boundary layer over the tube to reduce coherence of the vortices. For example, U.S. Pat. No. 7,836,780 to Garnett et al., which is assigned to Rosemount Inc., discloses the use of a helical flow modifying element. However, even with such approaches to vibration reduction, the sensor within the tube is still subject to loading sufficiently high to potentially damage the sensor after prolonged use. There is, therefore, a need to further reduce loading, particularly from vibration, on tubes such as those used in thermowells and averaging pitot sensors.